Process and apparatus for coating printed circuit boards

ABSTRACT

A process for coating printed circuit boards is described, comprising the following process steps: 
     the printed circuit boards on the coating side are kept at room temperature and, as appropriate, are heated to approximately 120° C.; then 
     in a first step, a preferably photopolymerisable, meltable, low-molecular-weight coating composition that preferably has an average molecular weight of from 500 to 1500 and that is highly viscous to solid at room temperature is coated in a thickness of approximately from 10 μm to 200 μm on to the surface(s) of the printed circuit board to be coated; 
     in a second step, a second, high-molecular-weight, photopolymerisable coating composition preferably having an average molecular weight of from 2000 to 10000 is applied in a thickness of from 2 μm to 20 μm over the first layer; and 
     the printed circuit boards so coated are cooled to room temperature and the two-layer photopolymerisable coating is exposed, preferably in contact with a mask, is developed and is cured fully. 
     An apparatus for carrying out the process outlined above is also described.

BACKGROUND OF THE INVENTION

The invention relates to a process for coating printed circuit boardswith a coating that is crosslinkable by electromagnetic radiation,especially UV radiation,. The invention relates also to an apparatussuitable for that purpose and to the use of the process for variouspurposes.

The function of the printed circuit board is to provide the conductiveconnection to the components. As miniaturisation progresses, the numberof connections is becoming ever greater, with the result not only thatprinted circuit board technology has produced multilevel circuitry, butalso that conductive tracks are becoming ever narrower, drill holediameters are becoming ever smaller and the number of conductive tracksbetween two holes is becoming ever greater.

With the development of surface-mounted devices it has been possible toachieve a further reduction in the surface area required. This has ledto conductor widths of less than 100 μm, to drill hole diameters of from0.3 to 0.2 mm and to solder point diameters of only up to 0.4 mm with upto seven conductive tracks between a drill hole grid of 2.54 mm. At thesame time, more and more connections must be made per integratedcircuit, which results in connection pad grids of from 0.3 to 0.4 mm.The problems resulting from the increasingly high integration densityare very complex and require a comprehensive solution. They begin firstwith the production of the conductive pattern.

For the production of a conductive pattern, the drilled copper-clad basematerial is coated with a resist. While such resists were in the pastscreen-printable etching or electro resists, nowadays photoresists arepredominantly used, which are solid or liquid resists. They are eitherlaminated on to the surface (solid resists) or applied by means of apouring machine or using rolls (liquid resists). After the applicationof a mask, the conductive pattern is fixed, for example, by exposure toUV light, with polymerisation of the resist, and exposed by development.This leads to the so-called pattern plating process. In pattern plating,first of all a mask is applied and only the conductor-free areas areexposed and developed. The conductive tracks are then built up byelectroplating and the drilled contact holes are clad with copper. Afterthe conductors have been deposited by electroplating, they are, forexample, provided with a tin coating, the electro resist is removed andthe conductive pattern is etched. Since the etching speed is the same inall directions, the undercutting corresponds approximately to thethickness of the copper film used. The undercutting that occurs in theetching stage of the pattern plating process represents the limit forlarge-scale application of that process. Moreover, the production ofconductors of equal height is frequently not possible as a result of thegeometry of the bath or of the printed circuit board.

Accordingly, for microconductor technology, the so-called panel platingprocess was developed. In that process, starting with the drilledcopper-clad base material, first of all the surface of the printedcircuit board and the drilled holes are clad with copper byelectroplating in order to achieve a uniform thickness of the copperlayer. A dry film resist is then laminated on, exposure is effectedusing a mask, and development is carried out.

In the case of microconductors, however, constrictions frequently occurbecause the line pressure of the laminating roller is unable tocompensate for uneven areas of the base material, so that the dry filmresist does not adhere to the same extent in all places. It isespecially important that the already copper-clad drill holes shouldalso be protected from the effects of the etching. That is achieved bycovering the holes with resist, so-called "tenting".

Further miniaturisation and the technology of surface-mounted componentshas led to so-called "rest-ring"-free through-holes. Dry film laminationtechnology cannot be used in this case, because without the so-calledrest-rings the resist film cannot be attached to the surface of theprinted circuit board. However, in order to be able to cover alsorest-ring-free holes with resist and thus protect them from the effectsof the etching, so-called electro-immersion coating was found, whichdeposits a resist film from 5 to 15 μm thick from a coating bath in thehole and on the surface of the printed circuit board. However, thatprocess is very cost-intensive and, because of the thinness of thecoating, can be used only together with the panel plating process.

A further problem of microconductors and fine pitch technology is thehigh connection density of the components and the associated connectiongrid of currently up to only 0.3 mm. In this respect, in particular theround surface of the solder applied to the connection pads in the hottinning process has a negative effect. The components frequently slideoff the round surface of the solder. As a solution to that problem,DE-A-41 37 045 proposes laminating a film of solder paste on to theentire surface of the printed circuit board provided with conductiveareas, exposing the printed circuit board with a positive mask to UVlight and exposing the connection points, the pads, by development.Solder paste is introduced into those openings in the resist using aknife. The solder paste that has been introduced is re-melted to formsolder deposits. Then the remaining film of solder paste is removedagain. Finally, the components are inserted into the printed circuitboard and soldered using the reflow soldering process. That process issuitable for small batch sizes. In the case of large-scale industrialmanufacture, however, it is uneconomical and results in markedadditional costs per square meter of printed circuit board. Moreover,air may be trapped between the conductive tracks when the printedcircuit boards are coated with the dry film of solder paste.

The problem underlying the present invention is to make available aprocess and an apparatus with which it is possible to produce coatingsof different thicknesses with photopolymerisable positive and negativeresists, to manufacture non-tacky, contact-exposable etching, electro,permanent and solder masks as well as masks for the production of highsolder deposits, and to close rest-ring-free drill holes in a reliablemanner. A further problem underlying the invention is to make availablea standard process with which it is possible to meet the demands thatare made by microconductor technology in respect of the quality of thecoatings. An economical and technologically variable process is to bemade available that is able to replace the many known processes based ondifferent physical principles. There is also to be provided an apparatusthat allows the process according to the invention to be carried out ina manner that is as cost-effective as possible and is qualitativelysuperior.

SUMMARY OF THE INVENTION

That problem and other associated problems are solved by a process forcoating printed circuit boards with a coating that is crosslinkable byelectromagnetic radiation, especially UV radiation, according to thelatter part of patent claim 1 and by an apparatus suitable for thatpurpose according to the preamble of patent claim 12. In particular, theprocess according to the invention for coating printed circuit boardscomprises the following process steps:

the printed circuit boards on the coating side are kept at roomtemperature and, as appropriate, are heated to approximately 120° C.;then

in a first step, a preferably photopolymerisable, meltable,low-molecular-weight coating composition that has an average molecularweight of approximately from 500 to 1500 and that is highly viscous tosolid at room temperature is coated in a thickness of approximately from10 μm to 200 μm on to the surface(s) of the printed circuit board to becoated;

in a second step, a second, high-molecular-weight, photopolymerisablecoating composition having an average molecular weight of approximatelyfrom 2000 to 10000 is applied in a thickness of from 2 μm to 20 μm overthe first layer; and

the printed circuit boards so coated are cooled to room temperature andthe two-layer photopolymerisable coating is exposed, preferably incontact with a mask, is developed and is cured fully.

By means of the two-step coating process according to the inventionusing a low-molecular-weight, meltable coating composition that ishighly viscous to solid at room temperature in the first step and ahigh-molecular-weight coating composition in the second step, it ispossible to produce on printed circuit boards coatings that arenon-tacky on the surface, so that the printed circuit boards can behandled further without difficulty and, in particular, exposure usingmasks in the contact process is possible. The two-step coating processis universally applicable; for example, it is in this way possible toproduce economically solder masks, etching or electro resists or alsopermanent resists for additive processes and masks for the production ofhigh solder deposits. The first partial layer of the coating does notnecessarily have to be non-tacky, since a high-molecular-weight layer isapplied in the second step, which ensures that the surface is non-tacky.The first coating step can be carried out using solvent-based liquidresists, but the use of low-solvent to solventless coating compositionsis preferred. Especially, the first coating composition is highlyviscous to solid at room temperature but can be melted at elevatedtemperatures without chemical bonds being destroyed. The first coatingcan be applied using the curtain pouring process or, alternatively, theroll coating process.

The high-molecular-weight second partial layer can be applied by variousmethods. However, the second layer is especially applied using thecurtain pouring process, by roll coating, by spraying, by lamination orby means of screen printing. After application of the second partiallayer, a total coating is obtained the surface of which is non-tacky andwhich has the required thickness and can be processed further in thecustomary manner.

The apparatus according to the invention for applying a coating that iscrosslinkable by electromagnetic radiation, especially UV radiation, tothe surface(s) of printed circuit boards has a number of processingstations for the printed circuit boards arranged along a conveyor beltfor the printed circuit boards. It comprises at least one coatingstation for applying the coating to the surface(s) of the printedcircuit boards, and a device for adjusting the temperature of thesurface(s) of the printed circuit boards to a temperature that ispreferably greater than or equal to room temperature. In particular, afirst and a second coating station are provided, the first coatingstation being arranged for the one- or two-sided application of apreferably photopolymerisable, meltable, low-molecular-weight firstcoating composition that has an average molecular weight ofapproximately from 500 to 1500 and that is highly viscous at roomtemperature, and the second coating station being equipped for theapplication, over the first coating, of a high-molecular-weight,preferably photopolymerisable, second coating composition having amolecular weight of approximately from 2000 to 10000. Thetemperature-control device for the surface(s) of the printed circuitboards is arranged upstream of the first coating station or forms partof that coating station.

Preferred variants of the process according to the invention and of theapparatus designed therefor, as well as various possible applications,are the subject of the dependent process and apparatus and use claims,respectively. The advantages of the variants will be seen from thedescription in conjunction with the diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its associated essential details are described ingreater detail below with reference to the drawings, in which:

FIG. 1 shows a diagrammatic view of an entire apparatus according to theinvention for the two-step coating of printed circuit boards,

FIG. 2 shows a first variant of a curtain pouring coating apparatus,

FIG. 3 shows a second variant of a curtain pouring coating apparatus,and

FIG. 4 shows a roll coating apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The coating apparatus for printed circuit boards shown diagrammaticallyin FIG. 1 comprises a first coating station 1 for applying a firstcoating composition and a second coating station 3 for coating with thesecond coating composition. The first coating station 1 comprises, forexample, two identically constructed curtain pouring apparatuses 10between which there is arranged a turning station 2 for turning theprinted circuit boards over. The second coating station 3 is, forexample, a spray coating station in which both sides of the printedcircuit boards are sprayed with the second coating compositionsimultaneously. Downstream of the second coating station 3 there isprovided a drying device 4 in which the printed circuit boards providedwith two coatings are dried to produce a non-tacky surface.

The coating apparatus shown customarily forms part of a larger printedcircuit board processing apparatus. Accordingly, there may be providedupstream of the first coating station 1, for example, drilling stationsfor producing drill holes in the printed circuit boards, mechanicaland/or chemical cleaning stations for cleaning the surfaces to becoated, or also electroplating stations. Downstream of the secondcoating station 3 or of the drying device 4 there are arranged, forexample, exposure stations in which the two-layer coating that has beenapplied is exposed preferably to UV radiation. Downstream thereof,etching stations may be arranged, for example, in which the exposedcoating is structured in the desired manner before the printed circuitboards are further treated in other stations. Before being coated withthe first coating composition, the surfaces of the printed circuitboards are stabilised at room temperature or heated to a temperature ofup to approximately 120° C. in a temperature-control device 5. Thetemperature-control device 5 is either in the form of a separate unitand arranged immediately upstream of the first coating station, forexample as in a roll coating apparatus (FIG. 4), or it forms part of thecurtain pouring apparatus, as is shown, for example, in FIGS. 1 to 3.

FIGS. 2 and 3 show in principle two preferred embodiments of curtainpouring apparatuses, which have been designed especially for theapplication of a photopolymerisable coating composition that is highlyviscous to solid at room temperature and is meltable at elevatedtemperature and that preferably has an average molecular weight of from500 to 1500. The curtain pouring apparatuses 10 shown by way of examplehave a heatable storage tank 11, 12 for the first coating composition,which tank is connected via a heat-insulated or, where appropriate,heatable connecting pipe 20 to a metering device 15, 16 or 22. By meansof the metering device, the first coating composition is applied to aheatable knife 17 or a heatable run-off plate 23 or the like, whichextends obliquely downwards from the metering device 15, 16 or 22 in thedirection of transport of the printed circuit boards and ends above thetransport plane of the printed circuit boards L. The angle ofinclination is preferably approximately from 300 to 600, especiallyapproximately 450, to the horizontal. A heating device 18 or a heatablecover 24 is arranged above the knife 17 or the run-off plate 23. Forexample, the heating device 18, as shown in FIG. 2, is an infraredradiator arrangement that is suspended in a pivotable manner in orderthat the distance of the infrared radiator from the knife 17 or therun-off plate 23 can be varied over their entire length.

The pouring table is indicated by two conveyor belts 7 and 8, betweenwhich a gap is left. A collecting vessel 19 for the first coatingcomposition is arranged in that gap beneath the transport plane of theprinted circuit boards L and is connected via a heat-insulated and/orheatable return pipe 21 to the storage tank 11, 12. Conveyor pumps arearranged in the connecting pipes 20 and in the return pipe 21, whichpumps are used to circulate the first coating composition. Heatexchangers 25 may also be provided in the return pipe, in order toadjust the first coating composition to the desired temperature.

In FIG. 2, the metering device is, for example, a heatable trough 16that is approximately V-shaped in cross-section and in which thererotates an applicator roll 15, which is likewise heatable. The liquefiedfirst coating composition is picked up by the applicator roll andscraped on to the knife 17. The metering device according to FIG. 3 is aconventional slot pouring device 22 with adjustable pouring lips. Theliquid first coating composition flows from the slot pouring device 22on to the heatable run-off plate 23.

The storage tank is divided into two chambers, an outer chamber 11 andan inner chamber 12, which communicate with each other in the vicinityof the base of the container. As is shown in FIGS. 2 and 3, the chambersare preferably arranged concentrically. The return pipe 21 for the firstcoating composition opens into the outer chamber 11, which is heatable,at the inlet opening 13. The inner chamber 12 preferably has at thelowest point of its base an outlet 14 for the liquid first coatingcomposition. The outlet 14 of the inner chamber 12 is arranged at alower level than the opening 13 of the return pipe 21 into the outerchamber 11 and is completely covered by the molten first coatingcomposition during operation. The special construction of the storagetank with two chambers 11, 12 allows the first coating composition to besucked out through the outlet opening 14 arranged at the lowermost pointof the base of the container without air bubbles being formed.

The meltable first coating composition that is highly viscous to solidat room temperature is preferably pre-heated in a warming cabinet andintroduced into the heatable storage tank 11, 12 at a viscosity ofapproximately from 5000 to 10000 mPas. In the storage tank, the firstcoating composition is heated to approximately 60° C. and is fed throughthe heat-insulated and/or heatable connecting pipe 20 to the meteringdevice 15, 16 or 17. From there, the first coating composition at atemperature of approximately 60° C. and a viscosity of approximatelyfrom 1000 to 5000 mPas is scraped on to the knife 17 or poured on to thesloping run-off plate 23. Because the knife 17 or the run-off plate 23is sloping, the first coating composition flows downwards. As the firstcoating composition flows over the knife 17 or the run-off plate 23,which are approximately from 20 to 50 cm long, it is brought to pouringtemperature and pouring viscosity. That is effected by means of theheating device arranged above the knife or the run-off plate, whichheating device is either an infrared radiator arrangement 18 (FIG. 2) oris formed by a heatable cover 24 arranged at a distance of preferablyapproximately 10 mm above the knife or the run-off plate (FIG. 3). As aresult, the first coating composition is heated to a pouring temperatureof approximately from 60° C. to 120° C. Its viscosity on pouring isapproximately from 100 to 500 mPas. The printed circuit boards L aremoved along at a speed of, for example, approximately from 100 to 200m/minute beneath the free-falling pouring curtain V that forms and arethus coated with the first coating composition, which has an averagemolecular weight of approximately from 500 to 1500, in a thickness ofapproximately from 30 to 200 μm. Before the printed circuit boards aretransported beneath the curtain, their surface to be coated is broughtto the required temperature in the temperature-control device 5. Thatmay involve, for example, stabilisation at room temperature, but it mayalso be desired to increase the temperature of the surface of theprinted circuit board to up to approximately 120° C. That is effected,for example, by means of an infrared radiator arrangement that isarranged above the first conveyor belt 7 of the pouring table, or abovea pre-accelerator belt 6.

Because the first, low-molecular-weight coating composition is broughtto the required pouring temperature and the required pouring viscosityonly immediately before being applied, it is ensured on the one handthat no chemical bonds decompose as a result of being subjected to heatfor too long, and on the other hand that the polymerisation reaction inthe first coating composition is not initiated unintentionally beforecoating. The first coating composition that flows down in the form of acurtain V is collected in a collecting vessel 19 and pumped back to thestorage tank 11, 12 again via the return pipe 21. A heat exchanger 25may be provided in the return pipe 21, by means of which the firstcoating composition can be adjusted to a temperature that corresponds tothe desired temperature in the storage tank of the first coatingcomposition in the storage tank 11, 12.

When the first side of the printed circuit board L has been coated inthat manner, the printed circuit board can immediately be transportedfurther to the second coating station 3, or alternatively it is turnedover in a turning station 2 in order that its second side can first becoated with the first, low-molecular-weight coating composition in anidentical curtain pouring apparatus.

In an alternative variant of the process according to the invention, thefirst coating of the printed circuit board is applied in a roll coatingapparatus. Depending upon the desired type of coating, one-sided ortwo-sided, the apparatus may be a single roll coating apparatus forone-sided coating, or two one-sided roll coating apparatuses can beconnected in series. As in the case of the curtain pouring apparatuses,a turning station is then arranged between the apparatuses. For thetwo-sided coating of printed circuit boards it is, however, preferred touse a two-sided roll coating apparatus 30, as is shown by way of examplein FIG. 4. The apparatus shown is in particular a two-sided roll coatingapparatus 30 that is equipped for the simultaneous coating of both sidesof the printed circuit boards and that has two heatable applicator rolls31, 33. The applicator surface of the applicator rolls 31, 33 ispreferably rubberised. Immediately adjacent to each applicator roll 31,33 there is arranged a metering roll 32, 34, which is likewise heatable.The metering rolls are so arranged that a narrow gap remains between themetering roll 32, 34 and the respective applicator roll. The width ofthe gap defines the thickness of the film of the first coatingcomposition that forms on the applicator roll 31, 33. For supplying thefirst coating composition there is preferably provided above theapplicator rolls 31, 33 a heatable storage container 35 for the coatingcomposition, from which heat-insulated or heatable supply pipes 36, 37lead to the respective pair of rolls 31, 32 and 33, 34. The upper andlower applicator rolls 31, 33 are preferably arranged at a distance fromeach other that corresponds approximately to from 50% to 95% of thethickness of the printed circuit board.

The metering gap between the rubberised applicator rolls 31, 33, whichare heated to from 60° C. to 90° C., and the metering rolls 32, 34,which are heated to from 70° C. to 110° C., is so set that a trough ofcoating composition is formed between the rolls. The first coatingcomposition, which is highly viscous to solid at room temperature, isliquefied in the storage container 35 to such an extent that it can befed to the two pairs of rolls 31, 32 and 33, 34. For coating, theprinted circuit boards L are transported through the gap between the twoapplicator rolls 31, 33 at a speed of approximately from 5 to 20m/minute. Because the applicator rolls and the metering rolls areheatable, it is possible to bring the first coating composition to therequired application temperature and the required application viscosityonly immediately before it is coated on to the surface of the printedcircuit board. Prior to coating, the printed circuit boards L arebrought to the desired coating temperature in a temperature-controldevice arranged upstream of the roll coating apparatus 30.

In a preferred process variant, the printed circuit boards L are heatedin a temperature-control device 5, prior to coating, so that thetemperature of their surface is higher than the temperature of thecoating composition applied. In this manner, the first coatingcomposition is liquefied even further on the surface of the printedcircuit board in order better to compensate for any uneven areas. Theapplication viscosity of the first coating composition in this preferredprocess variant is approximately from 1000 to 20000 mPas, preferablyapproximately from 8000 to 12000 mPas. The coating thickness applied isapproximately from 30 to 200 μm.

The drying device 4 arranged downstream of the roll coating apparatusis, for example, in the form of an air cooler, in which the coatedprinted circuit boards L are cooled to room temperature.

After application of the first coating composition, the printed circuitboards L are transported to the second coating apparatus 3, in which thesecond coating composition, which has a high molecular weight ofapproximately from 2000 to 10000, is applied over the first coatingcomposition. The second coating apparatus 3 may be a roll coatingapparatus, a spray coating apparatus, a screen printing coating station,a laminating station or a curtain pouring coating apparatus for the one-or two-sided application of the second coating composition. Thetwo-layer coating is then dried. The high molecular weight of the secondcoating composition ensures that the surface obtained is non-tacky afterdrying.

Depending upon the coating composition chosen, the two-layer coating issuitable for various applications. It has been found that inmicroconductor technology, economic considerations cannot be keyedprimarily to raw material, personnel and investment costs, but that itis the reject rate, or yield, that is the deciding economic parameter.This appears all the more pressing, the higher the degree of processing.Therefore, soldering errors on boards equipped with high-qualitycomponents have decisive significance. With the use of flux mediacontaining only small amounts of solid and the cessation of cleaning,problems have appeared during soldering. In wave soldering, solder beadsform, which adhere especially to smooth surfaces and can lead to shortcircuits. Since that phenomenon occurs especially on solder masksurfaces, but not with the base material, the reason was seen as beingthe rough surface of the base material produced by the treatment of thecopper film after etching. Although attempts to roughen the solder maskchemically and mechanically have been successful, the insulationproperties and the electrolytic corrosion of the coating are impaired asa result.

In order to eliminate that problem, fillers having a relatively largeparticle size are added to the second coating composition. Preferably,the proportion of fillers in the second coating composition isapproximately from 20 to 70% by weight. The average particle size of thefillers is approximately from 5 to 20 μm. In this manner, a surfacehaving a defined degree of roughness is obtained, which prevents solderbeads from adhering during the soldering process.

The two-layer coating may, however, also produce an etching or electroresist, which is removed again after the corresponding treatment of theprinted circuit boards. However, it may also give a two-layer permanentresist for additive processes, or be used as a mask for the productionof high solder deposits.

Examples of various two-layer coatings are described below. In theExamples, the following resin components are used for the preparation ofthe coating compositions:

Resin component A

A heat-curable resin that is crosslinkable by means of radiation and hasa viscosity of 200000 mpa·s at room temperature and a viscosity of 200mPa·s at 100° C. The resin is obtained by reacting 1 mol of atriglycidyl ether of cresol novolak having a molecular weight of 580with 1 mol of acrylic acid and then reacting the resulting product for 3hours at 60° C. with an unsaturated isocyanato carbamate ester having amolecular weight of 290, which has been obtained by reacting 1 mol oftoluylene-2,4- and -2,6-diisocyanate isomeric mixture with 1 mol ofhydroxyethyl acrylate in accordance with EP-A-0 194 360, Example 2.

Resin component B

Phenol novolak having a molecular weight of 250 and containingadditives, such as fillers, flow agents and curing accelerators, withthe following composition:

30 parts by weight of phenol novolak,

30 parts by weight of microtalcum,

30 parts by weight of aluminium oxide trihydrate,

6 parts by weight of ethylanthraquinone,

3 parts by weight of 2-methylimidazole, and

1 part by weight of flow agent Byk®361.

Resin component C

50% solution of a light-sensitive epoxy resin ofbis-1,3-(4-glycidyloxy-benzal)-acetone, bisphenol A andtetrabromobisphenol A, having a molecular weight of from 3000 to 3500,prepared in accordance with Example 1 of European Patent 0 075 537.

EXAMPLE 1

In a two-step coating process, a solder mask having a surface thatrepels solder beads is produced.

    ______________________________________                                        Printed circuit board:                                                                       base material FR 4, thickness                                                                   1.6 mm                                                      copper cladding:  17.5 μm                                                  conductor height: 50 μm                                     ______________________________________                                    

Coating composition 1

59 parts by weight of resin component A, and

41 parts by weight of resin component B.

The viscosity of the first coating composition is approximately 200000mPas at 25° C. The first coating composition is applied using thecurtain pouring process. The pouring temperature is approximately 100°C. The pouring viscosity is approximately 200 mPas. The surfacetemperature of the printed circuit board is adjusted to approximately30° C. The transport speed of the printed circuit board beneath thepouring curtain is set at 150 m/minute. The thickness of the first layerof coating is approximately 50 μm.

Coating composition 2

45 parts by weight of resin component C,

8 parts by weight of aluminium oxide,

5 parts by weight of talcum,

5 parts by weight of barium sulfate,

4 parts by weight of calcium carbonate,

9 parts by weight of magnesium oxide,

1 part by weight of phthalocyanine green, and

23 parts by weight of methyl glycol.

The second coating composition is applied in a thickness ofapproximately 5 μm using a spray coating apparatus and is dried withair.

EXAMPLE 2

Using the two-step coating process, a solder resist coating is appliedin a thickness greater than 100 μm. The coating is to be processed in afurther procedure for the preparation of deposit spaces over fine-pitchIC grids for receiving solid solder.

    ______________________________________                                        Printed circuit board:                                                                       base material FR 4 thickness                                                                    1.6 mm                                                      copper cladding:  17.5 μm                                                  conductor height: 65 μm                                                    IC grid:          0.3 mm                                       ______________________________________                                    

Coating composition 1 from Example 1 is applied as the first coatingcomposition using the roll coating process. The temperature of theapplicator roll is approximately 70° C. The viscosity of the firstcoating composition at 70° C. is approximately 1000 mPas. The surface ofthe printed circuit board has a temperature of approximately 120° C.during coating. The transport speed of the printed circuit board duringroll coating is 8 m/minute. The thickness of the coating applied isapproximately 100 μm.

Second coating composition: Resin component C

The second coating composition is likewise applied using the rollcoating process. The viscosity on coating on to the surface of theprinted circuit board is approximately 2000 mpas. The transport speed ofthe printed circuit boards during coating is approximately 10 m/minute.The thickness of the coating is approximately 10 μm. Then the printedcircuit board is dried in a vertical position in an air dryer. Once theprinted circuit boards are non-tacky at room temperature, they areexposed to UV radiation on both sides with the application of a mask,and the unexposed areas are dissolved away using a developer solutionand the coating that remains is cured fully. Solder paste is introducedinto the spaces that have formed over the IC connection pads and isre-melted at approximately 210° C.

EXAMPLE 3

Using the two-step coating process, an electro resist is applied, thefirst, meltable, low-molecular-weight coating composition being appliedusing the curtain pouring process and the high-molecular-weight secondcoating composition being laminated on in the form of a foil, as a dryfilm resist, by means of rolls.

    ______________________________________                                        Printed circuit board:                                                                       base material FR 4, thickness                                                                   1.6 mm                                                      conductor height: 65 μm                                                    drill hole diameter:                                                                            6 mm                                                                          (maxi-                                                                        mum)                                         ______________________________________                                    

First coating composition

Phenol novolak having a molecular weight of 500 and a viscosity of 20000mpa·s at 75° C. and 100 mPa·s at 120° C.

The pouring temperature of the first coating composition isapproximately 120° C., and its pouring viscosity is approximately 100mPas. The temperature of the printed circuit boards is adjusted toapproximately 80° C. The transport speed of the printed circuit boardsbeneath the curtain is approximately 170 m/minute. The coating thicknessobtained is approximately 50 μm.

Second coating composition 5: Dry photoresist

The printed circuit board coated on both sides with melt resist iscoated with a dry film photoresist in a second coating operation, inwhich a foil 25 μm thick is laminated on at approximately 40° C. using aroll laminator. Then the printed circuit board is exposed on both sideswith the application of a mask, and the unexposed areas are developed.The open areas can then be selectively plated with noble metals in thedesired manner.

What is claimed is:
 1. A process for coating printed circuit boards on acoating side with a coating that is crosslinkable by electromagneticradiation comprising the steps of:supplying at least one printed circuitboard with the coating side at a predetermined temperature; and thencoating a first layer of meltable, low-molecular-weight coatingcomposition that has a molecular weight of approximately from 500 to1500 and that is highly viscous to solid at room temperature in athickness of approximately 10 μm to 200 μm on to a surface of the atleast one printed circuit board to be coated; applying a second layer ofa high-molecular-weight composition having a molecular weight ofapproximately from 2000 to 10000, in a thickness of from 2 μm to 20 μmover said first layer; and cooling the at least one printed circuitboard so coated to room temperature and exposing, developing and curingthe first and second layers.
 2. A process according to claim 1, whereinthe first layer is coated at a temperature of approximately from 60° C.to 120° C. on to the surface of the at least one printed circuit board.3. A process according to claim 1, wherein the at least one printedcircuit board is coated on both sides prior to application of the secondlayer.
 4. A process according to claim 1, wherein the first layer isapplied using a curtain pouring process having a pouring viscosity, oncoming into contact with the surface of the printed circuit board, ofapproximately from 100 mPas to 500 mPas.
 5. A process according to claim4, wherein the first layer is applied from a storage container on to atleast one of a heatable run-off plate and a heatable knife that extendsobliquely downwards in a direction of transport of the at least oneprinted circuit board, and ends just above a transport plane of the atleast one printed circuit board, and is brought to pouring temperatureand pouring viscosity by heating devices arranged above the at least onepouring plate and the knife immediately before it is coated on to thesurface of the at least one printed circuit board.
 6. A processaccording to claim 1, wherein the first layer is coated on to thesurface of the at least one printed circuit board using a roll coatingprocess.
 7. A process according to claim 6, wherein an applicationviscosity of the first layer is approximately from 1000 mPas to 20000mPas, and a temperature of the surface of the at least one printedcircuit board to be coated is higher than a temperature of the firstlayer applied by a roll coating apparatus.
 8. A process according toclaim 6, wherein the first layer is applied simultaneously to pluralsurfaces of the at least one printed circuit board.
 9. A processaccording to claim 1, wherein the second, layer is applied using atleast one of a curtain pouring process, roll coating, spraying,lamination and screen printing.
 10. A process according to claim 9,wherein the second layer comprises approximately from 20 to 70% byweight, fillers having an average particle size of from 5 μm to 20 μm.11. A process according to claim 9, wherein the second layer is appliedto plural surfaces of the at least one printed circuit board.
 12. Theprocess according to claim 9, wherein said steps of coating and applyingfurther comprise a step of:forming said first and second layers as asolder mask on said at least one printed circuit board.
 13. The processaccording to claim 9, wherein said steps of coating and applying furthercomprise a step of:forming said first and second layers as a mask forproducing high solder deposits on said at least one printed circuitboard.
 14. The process according to claim 9, wherein said steps ofcoating and applying further comprise a step of:forming said first andsecond layers as at least one of an etching and an electro resist onsaid at least one printed circuit board.
 15. The process according to ofclaim 9, wherein said steps of coating and applying further comprise astep of:forming said first and second layers as a permanent resist foradditive processes on said at least one printed circuit board.
 16. AProcess according to claim 1, wherein said step of supplying furtherincludes the step of:maintaining the coating side of the at least oneprint circuit board at room temperature.
 17. A process according toclaim 1, wherein said step of supplying further includes the stepof:heating the coating side of said at least one printed circuit boardto approximately 120° C.
 18. A process according to claim 1, whereinsaid first and second layers are photopolymerisable compositions.
 19. Aprocess according to claim 1, wherein said first and second layers areexposed in contact with a mask.
 20. A process according to claim 7,wherein said application viscosity is approximately from 8000 mPas to12000 mPas.
 21. A process according to claim 11, wherein said secondlayer is applied to plural surfaces of the at least one printed circuitboard at the same time.
 22. An apparatus for applying a coating that iscrosslinkable by electromagnetic radiation to at least one printedcircuit board, said apparatus comprising:along a conveyor belt for theat least one printed circuit board, at least one coating station forapplying the coating to a surface of the at least one printed circuitboard; a device for adjusting a temperature of the surface of the atleast one printed circuit board to a temperature that is greater than orequal to room temperature; a first coating station that is arranged forat least one-sided application of a first layer of a meltable,low-molecular-weight first coating composition that has a molecularweight of approximately from 500 to 1500 and that is highly viscous atroom temperature; and a second coating station for application, over thefirst layer, of a second layer of a high-molecular-weight, secondcoating composition having a molecular weight of approximately from 2000to 10000, wherein the temperature-control device for the surface of theat least one printed circuit board is arranged at least one of upstreamof the first coating station and as a part of the first coating station;wherein the first coating station has at least one curtain pouringcoating device that comprises a storage container for the first coatingcomposition and at least one of a heatable run-off plate and a heatableknife that is connected to the storage container, extends obliquelydownwards in a direction of transport of the at least one printedcircuit board and ends just above a transport plane of the at least oneprinted circuit board, as well as heating devices, arranged above the atleast one of a run-off plate and knife, by which the first coatingcomposition flowing over the at least one of a run-off plate and knifeis brought to pouring temperature and pouring viscosity immediatelybefore it is coated on to the surface of the at least one printedcircuit board.
 23. An apparatus according to claim 22 wherein thestorage container has an outer and an inner chamber which communicatewith each other in a vicinity of a base of the storage container, theouter chamber being connected to a supply pipe for the first coatingcomposition and the inner chamber having an outlet for the first coatingcomposition, at the lowermost point of its base, the outlet of the innerchamber being arranged at a lower level than an opening of the supplypipe into the outer chamber and being completely covered by the firstcoating composition during operation.
 24. An apparatus according toclaim 22 wherein the first coating station is a roll coating apparatus.25. An apparatus according to claim 24 wherein the roll coatingapparatus further comprises two heatable, rubberised, applicator rollslocated opposite each other, between which there is an adjustable gap ofapproximately from 50% to 95% of a thickness of the at least one printedcircuit board, and which are supplied with the first coating compositionfrom a heatable molten composition storage vessel.
 26. An apparatusaccording to claim 22, wherein the second coating station is at leastone of a roll coating apparatus, a spray coating apparatus, a laminatingstation, a screen printing coating station and a curtain pouring coatingstation for the at least one-sided application of the second coatingcomposition.
 27. An apparatus according to claim 22, wherein said firstand second layers are photopolymerisable compositions.
 28. A coatedprinted circuit board, comprising:a first surface; a first layer ofmeltable, low-molecular-weight coating composition that has a molecularweight of approximately from 500 to 1500 and that is highly viscous tosolid at room temperature in a thickness of approximately 10 μm to 200μm formed on said first surface; and a second layer of ahigh-molecular-weight composition having a molecular weight ofapproximately from 2000 to 10000 in a thickness from 2 μm to 20 μmformed over the first layer.